This invention relates to a technique for growth doping, or growth by pulling from a melt, of single-crystal fiber devices.
The advent of low-loss glassy material optical fibers has created interest in fiber-compatible optical devices, many of which require crystalline materials. Nevertheless, a lack of single-crystal materials with the dimensions of fibers has been an obstacle to their realization. We describe here the adaptation of a pedestal growth (modified zone melting) technique, previously used for the growth of relatively large single-crystal fibers from refractory materials for applications requiring extreme strength, to the fabrication of a single-crystal Nd:YAG fiber laser. The technique obviously is not restricted solely to the growth of YAG fiber, but also may be used with suitable materials in the fabrication of a number of single-crystal fiber-geometry optical devices including, for example, other types of lasers, modulators, switches, Raman and Brillouin amplifiers, polarizers, and three-wave-interaction and parametric devices. The small cross section of fiber permits high optical power density for nonlinear effects without overheating, and also provides opportunities for phase matching through angle (mode) matching to obtain long interaction paths; thus a number of such devices for low-power continuous-wave applications should be feasible. In the antecedent single-crystal fiber growth that preceded the discovery of our present invention, single-crystal Nd:YAG fibers were grown as follows. A preform rod about 2 millimeters square, cut from a bulk sample of 1 percent Nd-doped YAG, was fed upward into the focused beam of a CO.sub.2 laser having sufficient power to melt the tip of the rod. A platinum wire 25-50 micrometers in diameter was dipped into the melt and raised slowly, pulling a crystal with an approximately round cross section from the melt. With appropriate differential feed rates for the source rod and the regrown crystal, stable growth of crystals with a diameter reduction of 2-3.times. from the original could be obtained. Successive regrowths have produced fibers with diameters as small as 50 micrometers and lengths to about 20 centimeters, limited only by our present equipment design. The required laser power ranged from 25-30 watts for the initial growth to 2-5 watts for the smaller sizes. At growth rates of approximately 0.5 centimeter/minutes, fibers smaller than about 250-micrometers diameter could be grown free of the "cracked" cores typically obtained in conventionally grown Nd:YAG. Larger fibers showed core cracking at this growth rate. Any imperfections in the fiber were highly visible during growth due to scattering of the light from the incandescent melt, and absence of visible cracking or inhomogeneities in the growing fiber could be taken as an immediate indication that a single-crystal fiber was being formed.
Nevertheless, fibers grown by that technique for laser purposes had to be grown from bulk rods which were already doped with the neodymium ion. The requirement of doping of the preform introduces an additional complication which we now recognize to be unnecessary. Indeed, more versatile techniques are desirable in order to obtain the optimum advantage from the new single-crystal fiber techniques and devices.